1. Field of the Invention
The present invention relates to a catalytic cracking process for converting C.sub.10 and higher olefin materials into more useful C.sub.9 and lower olefin materials having a high value in petrochemical and fuel applications.
2. Description of Related Art
One of the significant applications of C.sub.2 to C.sub.4 light olefins including propylene and butene-1, involves their conversion by pyrolysis or polymerization processes into higher olefin oligomers or to other products useful as high octane gasoline components. Incidental to such processes is the formation of minor amounts of olefin by-product materials having a predominant C.sub.10 and C.sub.11 olefin component with lesser C.sub.9 and C.sub.12 components, together with minor amounts of C.sub.13.sup.+ olefins.
These higher olefin by-product components are generally separated from the primary oligomer product and are collected for other uses. The composition of such collected by-products generally includes at least about 70 weight % of branched mixed C.sub.10 and C.sub.11 olefins, up to about 15 weight % of branched C.sub.9 olefins and up to about 20 weight % of branched C.sub.12.sup.+ olefins. Since it is difficult to further separate these materials into purer higher olefins and since these olefins are not as commercially valuable as are the lower olefins such as hexene, heptene, octene and nonene, this higher olefin mixture is normally distributed as a component for mogas or bunker fuel.
Various processes are known in the art for the catalytic cracking of heavier olefins into lighter olefins. For example, European Patent Publication 0109059 discloses the conversion of C.sub.4 to C.sub.12 olefins into propylene and lesser amounts of ethylene using modified or non-modified zeolite catalysts having a Si to Al ratio of 300 or less. Process conditions are space velocities higher than 50 kg/h, temperatures of 400.degree.-600.degree. C. and pressures slightly above atmospheric. The zeolite catalyst may be modified by ion exchange with compounds providing other ions such as ammonium ions. The preferred fractions to be cracked are butenes (Ex. 1-20), n-pentene-1 (Ex. 22-44) n-hexane (Ex. 45-48) and 2-methyl-1-pentene (Ex. 51 and 52).
European Patent Publication 0109060 discloses a similar process except that the zeolite catalyst is characterized as having a Si to Al ratio of greater than 350 and the catalyst may also include materials such as silicalites and their boron and chromium substituted polymorphs. Cracking conditions include space velocities of 5 to 200 kg/h, temperatures of 400.degree. to 600.degree. C. and pressures of slightly above atmospheric up to 8 atmospheres.
However, the processes disclosed in each of these references is highly selective towards the formation of C.sub.3 and C.sub.4 compounds, largely as a consequence of the composition of the feedstock and the relatively high cracking temperatures and space velocities employed in the process.
Abbot et al. in the Canadian Journal of Chemical Engineering, Vol. 63, June 1985, teaches the catalytic cracking of substantially pure n-alkenes (hexenes through nonenes) at 405.degree. C. using a Zeolite ZSM-5 catalyst. The reference indicates that olefins smaller than C.sub.6 are stable to cracking and must first dimerize before a species is formed susceptible to cracking, whereas higher olefins up to nonenes are subject to monomolecular cracking to yield mixed lower olefins, such as mixtures of two or more of C.sub.3, C.sub.4, C.sub.5 and C.sub.6 olefins. This process is selective towards the formation of C.sub.6 and lower olefins largely as a function of the cracking temperatures and composition of the relatively pure olefin feedstock.
Zeolite materials have long been known in the art and have been used as cracking and petrochemical conversion catalysts, as well as molecular sieves for molecular separation. The materials and applications of Zeolites are disclosed in various publications: Barrer, "Zeolites and Clay Minerals as Sorbents and Molecular Sieves", Academic Press, London, 1978; Breck, "Molecular Sieves", (1974), J. Wiley, (New York). They may be characterized as crystalline aluminosilicates having a rigid three dimensional network of SiO.sub.4 and AlO.sub.4 in which the tetrahedra are crosslinked by the sharing of oxygen atoms whereby the ratio of the total aluminum and silicon atoms to oxygen is about 1:2. The electrovalence of the tetrahedra-containing aluminum is balanced by the inclusion in the crystal of a cation such as an ammonium ion, an alkali or alkaline earth metal ion or mixtures thereof, or by a hydrogen ion in the acid form.
U.S. Pat. No. 3,140,249 discloses a zeolite type catalyst which has been modified by ion exchange to replace the alkali metal ion with another ion. The catalyst is disclosed to be useful in cracking hydrocarbon oils to lighter materials boiling in the gasoline range under cracking temperatures of at least 700.degree. F.
U.S. Pat. No. 3,702,886 discloses a zeolite catalyst designated as "ZSM-5" useful, inter alia, for cracking hydrocarbons. The catalyst may be useful in the alkali metal form or exchanged with ammonium or a different metal.
U.S. Pat. No. 3,709,979 discloses zeolite ZSM-11 which may be used for cracking hydrocarbons. The catalyst may be used in the alkali metal form or may be ion exchanged.
U.S. Pat. No. 3,308,609 teaches a method for the production of a zeolite referred to as Zeolite beta by forming the crystallization product from an aqueous reaction mixture comprising amorphous silica, a soluble aluminate and tetraethylammonium aluminate. The catalyst may be used in the alkali metal form or may be ion exchanged using a different metal.
"Silicalite" may be characterized as a near pure silica form of ZSM-5. In this context, the term silicalites also includes various intergrowth forms, alternately called "pentasils" in the literature such as disclosed in U.S. Pat. No. 4,229,424 and by Kokotailo et al., Chemical Society Special Publication #33, Ed. R. P. Townsend, p. 133 (1980).
Such materials are referred to in the aforementioned E. P. 0109060. In addition, U.S. Pat. No. 4,061,724 discloses the preparation of "silicalite" by a hydrothermal crystallization of a reaction mixture comprising water, a source of silica and an alkylammonium compound, followed by calcination. This material is a ZSM-5 type with a very high content of SiO.sub.2.
These and other catalyst systems and processes are particularly useful in cracking hydrocarbon oils and in numerous petrochemical processes. While these are useful products and processes, it would be particularly desirable to develop a process for cracking C.sub.10.sup.+ higher olefins which demonstrates selectivity towards the production of olefins in the C.sub.6 to C.sub.9 product fraction boiling within the range of 20.degree. to 154.degree. C., and particularly a process which leads to enhanced yields of hexene, heptene, octene and nonene while minimizing the production of C.sub.2 to C.sub.4 olefins and saturated hydrocarbons. These C.sub.6 -C.sub.9 olefins are particularly useful as feedstocks in the production of alcohols, alkyl phenols, acetates and like chemicals which have numerous industrial applications such as surfactants, plasticizers, resin-forming reactants and the like.